Copyright infringement is a serious issue in the recording industry and among musicians. What some consider “sampling,” others consider stealing. Yet sampling is a lucrative path to creating music. The infringer does not need to re-perform a sampled piece and does not need to pay others to recreate it. Naturally, musicians and record labels wish to protect their music from being sampled and re-used wholesale by infringers.
In “music minus one” recordings, a mix of a song is provided minus one musical part, e.g., a guitar track or a vocal track. These music-minus-one recordings are useful for training because the user can practice the missing audio and have a full accompaniment that can be rewound or replayed as often as necessary.
A danger of music-minus-one recordings is that if the music-minus-one recording is “subtracted” from the original recording (with all parts present), the “stem” (track) that represents the missing audio can be recreated. This will give the infringer a perfect copy of the missing track. This is especially an issue for recent rhythm-action games because they can, in theory, be music-minus-one recordings for each part—guitar, bass, vocals, and drums—as well as full recordings (assuming all parts are played perfectly).
A typical rhythm-action game mixes together individual stems during gameplay. To make the experience immersive, the game can mute, attenuate, or otherwise alter a stem when the corresponding player's performance is imperfect. The game can also mix in additional sounds to indicate that a mistake was made. When a player provides the expected input at the expected time, however, their corresponding stem is unmuted, unattenuated, and otherwise unaltered in the final mix.
Selective muting or attenuation allows a potential infringer to derive a particular audio track by playing the rhythm-action game first perfectly (so the whole ensemble is heard) and then again, but missing every note. If both performances are recorded, the all-misses recording can be subtracted from the all-perfect recording and the stem for that part can be obtained. The following example illustrates this type of attack.
An example song is comprised of a stem audio track “X” and an additional audio track “A.” In some embodiments, A is the playback of all instrument tracks except stem audio track X, while in other embodiments, A is the composite of several separate stem tracks, each representing an instrument or combination of instruments. During a first play-through of the song, a player uses a video game controller to provide input that matches musical cues (known as gems) corresponding to stem audio track X, such that the video game platform outputs audio that is an exact reproduction of the full ensemble in the original song, i.e., A+X. During a second play-through of the song, the player provides no input to match the musical cues corresponding to stem audio track X, such that the video game platform outputs an audio track that is the full audio of the original song, but without the stem represented by stem audio track X. Subtracting the audio output of the second play-through from the audio of the first play-through results in the stem audio track X. In other words, (A+X)−A=X. Using this technique, valuable stem tracks can be stolen.